HDU 2242 road-air-conditioned classrooms (dual-connection)

HDU 2242 road-air-conditioned classroom

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Idea: Calculate the edge dual-connected component and then scale down the vertex. The vertex weight is the sum of the vertex weights of the dual-connected branch. After the point is scaled down, it is changed to a tree. Then the answer can be obtained through the DFS on the tree.

Code:

#include <cstdio>#include <cstring>#include <cstdlib>#include <algorithm>#include <vector>using namespace std;const int N = 10005;const int M = 20005;int n, m, val[N];struct Edge {int u, v, id;bool iscut;Edge() {}Edge(int u, int v, int id) {this->u = u;this->v = v;this->id = id;this->iscut = false;}} edge[M * 2], cut[M];int en, first[N], next[M], cutn;void add_edge(int u, int v, int id) {edge[en] = Edge(u, v, id);next[en] = first[u];first[u] = en++;}int pre[N], dfn[N], bccno[N], bccval[N], bccn, dfs_clock;void dfs_cut(int u, int fa) {pre[u] = dfn[u] = ++dfs_clock;for (int i = first[u]; i + 1; i = next[i]) {int v = edge[i].v;if (edge[i].id == fa) continue;if (!pre[v]) {dfs_cut(v, edge[i].id);dfn[u] = min(dfn[u], dfn[v]);if (dfn[v] > pre[u]) {edge[i].iscut = edge[i^1].iscut = true;cut[cutn++] = edge[i];}} else dfn[u] = min(dfn[u], pre[v]);}}void find_cut() {dfs_clock = 0; cutn = 0;memset(pre, 0, sizeof(pre));for (int i = 0; i < n; i++)if (!pre[i]) dfs_cut(i, -1);}void dfs_bcc(int u) {pre[u] = 1;bccno[u] = bccn;bccval[bccn] += val[u];for (int i = first[u]; i + 1; i = next[i]) {if (edge[i].iscut) continue;int v = edge[i].v;if (pre[v]) continue;dfs_bcc(v);}}vector<int> bcc[N];void find_bcc() {bccn = 0;memset(bccval, 0, sizeof(bccval));memset(pre, 0, sizeof(pre));for (int i = 0; i < n; i++) {if (!pre[i]) {dfs_bcc(i);bccn++;}}}const int INF = 0x3f3f3f3f;int ans, tot;int gao(int u, int fa) {int sum = bccval[u];for (int i = 0; i < bcc[u].size(); i++) {int v = bcc[u][i];if (v == fa) continue;int tmp = gao(v, u);sum += tmp;ans = min(ans, abs(tot - 2 * tmp));}return sum;}int main() {while (~scanf("%d%d", &n, &m)) {en = 0;memset(first, -1, sizeof(first));tot = 0;for (int i = 0; i < n; i++) {scanf("%d", &val[i]);tot += val[i];}int u, v;for (int i = 0; i < m; i++) {scanf("%d%d", &u, &v);add_edge(u, v, i);add_edge(v, u, i);}find_cut();find_bcc();if (cutn == 0) {printf("impossible\n");continue;}for (int i = 0; i < bccn; i++) bcc[i].clear();for (int i = 0; i < cutn; i++) {int u = bccno[cut[i].u];int v = bccno[cut[i].v];bcc[u].push_back(v);bcc[v].push_back(u);}ans = INF;gao(0, -1);printf("%d\n", ans);}return 0;}

HDU 2242 road-air-conditioned classrooms (dual-connection)